In the control of power being transmitted by rotatable shafts in various types of equipment, there is a need to continuously measure the static or dynamic torque by an accurate, compact, and low cost device on an integral shaft. One such need is to measure the torque in the drive shaft of an automobile in order to automatically control the power delivered to the wheels.
Optical methods have been disclosed in the prior art which measure the torque at a coupling between two shafts or the torque in an integral shaft.
U.S. Pat. No. 2,811,853, Friedman relates to a device for visual measurement of torque which is essentially a coupling inserted between a drive shaft and a load shaft and is thus not applicable to measuring torque on an integral shaft.
U.S. Pat. No. 3,163,037, Kawabata discloses a device for visual measurement of torque in which the coupling of the drive shaft and load shaft is achieved by use of a torsion bar and thus also is not applicable to measuring torque on an integral shaft. In the patent to Kawabata, the optical members are coaxial cylindrical shells, each attached at one end to the shaft member whose relative angular deflection is to be sensed. Each shell has a longitudinally extending slit intersecting at an angle to define a visual intersection point whose axial position is a function of the angular deflection of the shaft under torsional load. The system operates with a shutter or stroboscopic illuminating means synchronized with the rotational speed and thus cannot be used for static torque measurements. It also has inherently low resolution because of the low information sampling rate and the lack of any form of torque magnification such as can be obtained by moire fringe patterns as in U.S. Pat. No. 2,938,378 of Canada et al.
In the latter patent, the optical members are radially extending disks, each having diffraction gratings thereon. As applied torque causes one of the optical gratings to rotate relative to the other, a pattern of moire fringes is produced. The alternate bands of light and dark fringes passing through a light beam as the shaft rotates are used to modulate the light beam. Photoelectric detection means and processing circuitry are used since the system is not suitable for visual reading. When the shaft is rotating at constant torque, the moire fringe bands are moving past the photoelectric detection means with a frequency proportional to shaft rotational speed. The frequency measured is related to the product of rotational speed and torque, which is horsepower. Thus, to measure torque independently, would require additional circuitry to independently measure rotational speed and factor this effect out of the horsepower measurement. The apparatus of Canada et al is not suitable for measurement of static torque in a non-rotating shaft. This is a serious limitation for application to shafts in vehicles where a measurement of torque is required starting from zero shaft rotation. The system in addition to being bulky in a radial direction, suffers the further limitation that rapid torque transients cannot be measured because of the sampling at intervals by shutter or stroboscopic means.
Some of the aforementioned limitations are avoided in U.S. Pat. No. 3,688,570, Burke, for an angular deflection meter for measuring static or dynamic torque. The optical means are rigidly attached at spaced positions on an integral shaft to measure the relative angular displacement. The optical means consists of two coaxial cylinders as in Kawabata but differs in utilizing the principle of moire fringes as in Canada et al. Burke teaches that the grid pattern on each cylinder should be helical in different directions on each cylinder to create a moire fringe pattern. Relative angular deflection of the cylinders under static or dynamic load causes the circumferential interference moire fringes to move in an axial direction in proportion to the applied load. A small angle .theta. between the grid lines on the respective cylinders enhances visibility and maximizes the magnification of the system. The axial movement of the circumferential moire fringes can be observed visually or by an optical system using a light source to illuminate the fringes and including a photodetector and signal processing circuit. Movement of fringes past a slit in a mask in a plane perpendicular to the shaft axis modulates the light received by the photodetector. To determine the direction of torque, additional optical equipment is required involving a neutral density optical wedge. The method disclosed by Burke for determining the direction of torque is complex and requires complex signal processing circuitry and additional components for the optical measurement means. The present invention provides much simpler devices offering novel improvments over the prior art.